This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        DRX Discontinuous Reception        E-UTRAN Evolved UTRAN        HetNet Heterogeneous Network        HO Handover        HW Hardware        ISD Inter-Site Distance        PDCCH Physical Downlink Control CHannel        Rel Release        RF Radio Frequency        RLF Radio Link Failure        RRC Radio Resource Control        RSRP Reference Signal Received Power        RSRQ Reference Signal Received Quality        SW Software        TS technical standard        UE User Equipment        UTRAN Universal Terrestrial Radio Access Network        
E-UTRAN mobility in RRC connected mode contains certain challenges, as the mobility concept is based on connected mode mobility as this was defined in UTRAN. E-UTRAN mobility in RRC connected mode only supports UE-assisted network controlled mobility by use of a hard handover. This means that the mobility basically is based on the network configuring the UE with a given measurement configuration, which the UE is then required to follow according to requirements specified in 3GPP TS 36.331 and 3GPP TS 36.133, i.e., the UE performs the measurements as instructed and sends measurement reports to network according to given configured events. The network may then use the received measurement reports for initiating mobility, e.g., causing the UE to initiate handover from a current cell to stronger neighbor cells, based on the received measurement report. The basic approach in the early definition of E-UTRAN mobility was that the mobility was to be optimized for intra-frequency mobility in a homogeneous macro deployment.
Introduction of connected mode power savings through the use of DRX has lead to less frequent measurement sample requirements for the UE in order to enable full advantage of the power saving options (i.e., the UE is allowed to take fewer mobility measurement related samples when applying DRX than if continuous reception is ongoing). This approach means that even in a macro-cell layout, care has to be taken from the network side in order to ensure that the network configures the UE with mobility related parameters that are suitable for the applied DRX configuration. In general, this is a challenge for long DRX cycles in homogeneous macro layer deployment, but it is possible.
Adding a HetNet scenario to this situation adds to the challenges of mobility in E-UTRAN RRC connected mode. A heterogeneous network is a network containing cells with different characteristics such as transmission power and corresponding RF coverage area. Typically, a HetNet contains macro cells with a larger RF coverage area, and small cells with smaller RF coverage areas generally inside the larger RF coverage area of the macro cells. The smaller cells have names based typically on their RF coverage areas, such as micro, pico, and femto cells (from the largest area of the micro cell to the smallest area of the femto cell). The HetNet scenario therefore requires consideration for handover and DRX of small cells and macro cells, further adding challenges to mobility in, e.g., E-UTRAN RRC connected mode.